Light sources are critical components in many analysis systems. The most common types are lamps such as Xenon, Mercury, Deuterium and Tungsten-Halogen lamps. These lamps are located within the analysis system alongside a detection unit, usually a semiconductor-based or photomultiplier-tube detection device. Typical modes of operation for these optical systems include absorption, reflection, scattering and fluorescence monitoring.
These systems are used in applications as diverse as chemical and materials analysis, medical diagnostics, bio-sensing and process monitoring. These are large established markets that are growing in size, such as in the field of medical diagnostics, that require portable or point of care diagnostics.
These lamps are considered to be specialty or high-value products when compared to lamps used for illumination or display. They emit a broad and continuous optical spectrum, often with a large component in the Ultra-Violet (UV) region, which is important for many sensing applications. They are also relatively difficult to manufacture and as a result, they often display variations or inconsistencies in performance which is a common problem reported by many users.
On top of this, the lamps have limited lifetimes, often of only a few hundred hours, and require careful handling such as the use of special power supplies and temperature control systems. However, even though these lamps can present such problems, they are used extensively and are considered a mature technology. To-date there has been no real effort to replace them, mainly due to the lack of a viable alternative.
Light Emitting Diodes (LEDs) have become a mature illumination technology over the past decade. They are used in many illumination and display applications and the cost benefits of implementing the LED option is becoming more attractive. The cost per lumen of LED products (a common price benchmark for lighting systems) is now approaching those of conventional lamps and their operating lifetimes can be an order of magnitude higher. Furthermore, where improvements in conventional lamps have stalled, LED technology is continuously improving (increased brightness and lower costs) and their business case becomes more compelling.
However, the main problem with LEDs is that they emit a relatively narrow optical spectrum and exhibit uneven levels of intensity across the optical spectrum. This is not a problem for most illumination and display applications, as the spectrum can be easily tailored to match the response of the eye. However, it is an issue for analysis systems that require a broad and continuous optical spectrum. In addition, developments in UV devices have to-date been slow, mainly due to the lower commercial demand relative to lighting and display markets. However, UV LEDs are now commercially available and their output powers meet the requirements for some applications. Currently, there is no effective alternative to conventional lamps for a wide spectrum optical source. The major problems associated with these sources are well documented and include:
Highly inefficient, generating large amount of heat.
Expensive and inconsistent optical performance.
Very short lifetimes and must be replaced regularly which can affect optical performance and mis-alignment issues.
Require complex power supplies and thermal management systems.
Can be dangerous (toxic, explosive and risk of burns and eye damage).
Not suitable for portable or handheld systems due to size restrictions and significant electrical power requirements.
A possible solid-state solution that currently exists is based on large area LED arrays. However, this simple technology is not suitable because the number of LEDs required to encompass the optical spectrum requires a large area device (typically 0.5 cm to 1.0 cm wide). This results in significant spatial or position dependence on emission wavelength, which is unacceptable for coupling into small apertures, such as spectrometers or optical fibres. In addition LED arrays on the market are usually monochromatic or comprised of a limited number of wavelengths, typically red, green and blue.
U.S. Pat. No. 5,109,447 discloses a high-powered, spectrally flat, very broadband optical source including optical coupler and method using same. This patent describes an array of LEDs coupled to a series of optical fibres using micro-lenses. U.S. Pat. No. 6,242,857: High efficiency fluorescent lamp with low colour rendering property. This patent is typical of the concept to use a UV or blue LED chip to excite a phosphor, with the combined emission providing a white or broadband colour. This general concept is widely used to create white light LEDs. US Patent publication number US2006/0267037A1 discloses a Light Emitting Diode Package. This patent publication describes a LED array of different colours and micro-lenses to diffuse and mix the colours to produce an even illumination pattern. Other patent publications in the art include US 2004/156199 (Rivas Nelson et al); US2003/223236 Wu Jiahn-Chang); GB 2 374 919 (Altman Stage Lighting Co Inc); CA 2570 234 (Erco Leuchten) and US 2004/165379 (Waters Ryan).
However none of these publications effectively provide an efficient wide spectrum light source. There is therefore a need to provide a wide spectrum light source that overcomes the above mentioned problems.